Keep your battery in good nick and it will light up your evenings. Darryl Goodier explains how to keep the current flowing.

Getting started Hidden in a corner of your tourer is an often misunderstood device that is responsible for much of your caravanning comfort. It's your battery charger. Despite its low-key appearance (it lacks lights and user controls) this is an essential piece of kit that provides a charge to the caravan's leisure battery, and supplies power to all your 12V appliances and lights.
Typically it will be located in a bed box or wardrobe sited adjacent to the mains and 12V fuse boxes. It needs little care or attention and barring external influences (such as water seeping in) it should last a lifetime.
But the charger is wedded to the leisure battery, which can be fickle and may require differing levels of attention depending on its state of charge. Your existing charger might be perfectly adequate if your battery meter always points at nearly or fully charged, but not if a battery is 'flat' or even very low on charge.
The device fitted as standard to most caravans is not actually a true battery charger, but more akin to a transformer that converts 230V AC to 12V DC and gives out a constant charge of around 13.5V. This is fine for a battery that's partly discharged and regularly used, but bad news for a battery that has lost all its charge, or one that is worked hard and drained regularly – for instance, if you often use an axle-mounted caravan mover that draws on the leisure battery.
Give a flat or low-charged leisure battery a steady charge and you're heading for trouble. Not only will it never reach the state of 'full charge', but you will damage it in the process. The lead plates inside the battery will become coated with deposits of lead sulphate, a coating that prevents the plates from taking a full charge. This, in turn, leads to further sulphation and the process spirals until your battery dies. It's a bit like a kettle element furring up with limescale.

What's so special about a leisure battery?
Leisure batteries and car batteries do the same basic job, but they do it completely differently. A car battery provides a very short but massive burst of effort when starting the engine, followed by long sessions of charging while the engine is running. This is known as 'shallow-cycling', which normally ensures the car battery is almost always fully charged.
Leisure batteries rarely do any hard work and spend most of their lives dormant, supplying low levels of current, or 'flat', which is known as 'deep-cycling'. This almost sedentary lifestyle is the Achilles' heel of leisure batteries and, in response, their manufacturers have devised ranges to meet the varying needs of caravanners. These include, at one end of the spectrum, traditional fluid lead-acid batteries that cope well with rapid charging. At the other end are batteries with gel fillings for long periods of storage.

What is the best way to charge a battery?
What's needed is a staged charger that senses the state of charge of the battery and supplies the voltage that best suits that condition. There are several models from which to choose, ranging from a decent two-stage charger at around £80, to complex multi-stage chargers that use state-of-the-art circuitry to ensure optimum performance of the charger and battery, but these can cost hundreds of pounds.
A two-stage charger will sense the battery's state and, if its charge level is above a pre-set level it will supply the 'normal', or 'float', charge of 13.8V. But if it detects that the battery charge level is below the pre-set level, or is fully discharged, it will start charging with a 'kick-start' at over 14V and automatically switch back to 13.8V once it has detected the battery charge has risen out of the danger-zone. This initial boost prevents the build up of sulphate deposits. Overall, it takes four to six hours for the battery to hold a credible charge.
The root of the problem is money. Caravan manufacturers, who strive to keep production costs down, fit a 'charger' that does the job for the least cost. (Dealers then fit the batteries.) But, with a decent 110Ah leisure battery now costing well over £100, spending a few extra quid on a better charger is a good idea. Once your battery fails, most battery retailers will take another £100 off you without exploring or explaining the cause. However, change normally only comes in response to consumer pressure and certain makers are looking at improvements in this area.
For the time being, the only option for most of us is to change the existing constant-voltage charger for a newer, staged charger. Several are on the market, costing from around £80, and they're relatively easy to fit (see p137). If you're not competent with electrics, your dealer will fit it for £30-£50.